This invention relates to the detection of objects with given, known characteristics against a background. The problems associated with the detection and identification of for example vessels against the sea surface has interest in this connection. Primarily the invention has been developed for radar systems and the employment of electromagnetic waves, but obviously it may also be applied in sonar and the like which is based upon acoustic waves.
In radar systems which include powerful data processing equipment the invention can give a monitoring capability which has been quite impossible hitherto, by employing corresponding optimal radar structures and signal processing algorithms.
It is known in radar systems to transmit coherent electromagnetic waves having characteristics adapted to the object or objects, background wave pattern or other wave patterns to be brought forth, or possibly suppressed in the detection. Such adaption may be designated transmitter generated filter functions. The use of filter functions at the transmitter side involves that after an adapted detection at the receiver side there is obtained a decision as to whether an object corresponding to the filter function concerned, is present. The shape of the object in the space domain as well as its movements in the time domain are independent parameter sets which may be analysed. A hologram-like interference pattern of a wave or vessel structure or both is put on the sea surface by an outgoing wave and a reflective wave interfering and forming standing waves. Then the hologram or interference pattern can be swept so as to investigate which specific propagation directions or orientations in space are present. Because there exist relationships between for example the movement of vessels and associated water waves induced on and in the water, a wave hologram and a vessel hologram can be established on the sea surface simultaneously in order to enhance the detection process.
It is clear that what is an "object" and what is "background", is purely a question of definition. Thus, for example the object of interest may possibly be a "naturally" occuring water wave pattern, whereas vessels and accompanying wave patterns on the water surface may be regarded as an interfering background or noise, which the detection process attempts to suppress.
For a closer description of the more advanced radar systems of the recent time, being of interest in connection with the above and forming the background of this invention, reference may be made to the following:
1. Gjessing, D T, 1977, "A Generalized Method for Environmental Surveillance by Remote Sensing", Radio Science, Vol 13, No 2. PA0 2. Gjessing, D T, 1978, "Remote Surveillance by Electromagnetic Waves for Air-Water-Land", Ann Arbor Publishers Inc, Ann Arbor, USA. PA0 3. Gjessing, D T, 1981a, "Adaptive Radar in Remote Sensing", Ann Arbor Publishers Inc, Ann Arbor, USA. PA0 4. Gjessing, D T, 1981b, "Adaptive Techniques for Radar Detection and Identification of Objects in an Ocean Environment", IEEE Journal of Ocean Engineering, OE-6.1, 5-17. PA0 5. Gjessing, D T, 1979, "Environmental Remote Sensing. Part I: Methods Based on Scattering and Diffraction of Radio Waves", Phys Technol, Vol 10. PA0 6. Gjessing, D T, Hjelmstad, J, Lund, T, 1982, "A Multifrequency Adaptive Radar for Detection and Identification of Objects. Results on Preliminary Experiments on Aircraft against a Sea Clutter Background", IEEE Transactions on Antennas and Propagation, AP-30, 3, 351-365. PA0 7. Gjessing, D T, Hjelmstad, J, 1982, "Adaptive Radar in Remote Sensing Space, Frequency and Polarization Processes", Proc IEE Radar 82, London, October 1982. PA0 8. Gjessing, D T, Hjelmstad, J, Lund, T, 1983, "Directional Ocean Spectra as Observed with a Multi-Frequency CW Doppler Radar System", Int J Remote Sensing 1984, 5, 2. PA0 9. Bass, F B, Fuks, I M, 1979, "Wave Scattering from Statistically Rough Surfaces", Pergamon, N.Y., PA0 10. Dysthe, K, 1980, "Havbolger og fysikk", Fra Fysikkens Verden.